(A student is introducing herself and her sister.)

Hi. I am Yujin. I am from Korea.
I go to middle school. I have a sister. She is very pretty. Her name is Yuri. Bye.

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Is it OK to use 'Bye' after the introduction? Do we have to use other expressions?

No, we don't say "bye" after an introduction.

A smoother way to say this would be
"My name is Yujin. I am a middle school student from Korea. I have a very pretty sister named Yuri. I am pleased to meet all of you."

It is generally more common to use a different expression rather than simply saying "Bye" at the end of an introduction. While it's not necessarily wrong to use "Bye," it can come across as abrupt or informal. Here are a few alternative expressions you could use:

1. "Nice to meet you": This is a polite and common way to end an introduction. It expresses your pleasure in meeting the person you are introducing yourself to.

2. "It was great meeting you": This is another friendly way to conclude an introduction, expressing the positive experience of meeting the person.

3. "I look forward to getting to know you better": If you want to convey your interest in establishing a relationship beyond the introduction, this expression can be used.

Remember that the context and your level of formality should guide your choice of expression.